A semiconductor laser includes a multi-layer structure having an active layer sandwiched by cladding layers having a bandgap energy wider than that of the active layer, and a pair of laser resonator facets facing each other and being perpendicular to the multi-layer structure. Recently, demands for semiconductor lasers providing higher light output power is increasing in order to improve the performance of optical information processing systems and optical communication systems. The maximum light output power is limited to some extent due to the occurrence of catastrophic optical damage, which will be described later, for semiconductor lasers with lasing wavelength less than 900 nm, such as short wavelength lasers and visible lasers. The catastrophic optical damage occurs at the active layer in the vicinities of the resonator facets in which the temperature increases locally by absorption of a part of light output power.
Two methods to avoid the occurrence of the catastrophic optical damage were reported previously. The first method is decreasing a laser light density at the active layer by expanding the distribution of the laser light to the outside of the active layer. The second method is decreasing the light absorption coefficient of the active layer in the vicinities of the resonator facets.
For instance, a large optical cavity structure, which have (an) optical waveguide layer(s) between an active layer and cladding layers, expands the laser light distribution in a direction perpendicular to the active layer, and effectively decreases the laser light density at the active layer. Further, it has been reported that a non-absorption mirror structure, which includes a semiconductor layer having a bandgap energy wider than that of an active layer in regions of resonator facets and the vicinities thereof, and a window structure, in which an active layer is p-type in an active region, and n-type of a high carrier concentration in the vicinities of resonators facets, are effective to decrease the absorption coefficient of the active layer in the vicinities of the resonator's facets, and increase a light output power limit. Still further, a high output semiconductor laser including an active layer of a quantum well structure has been described in "The light amount IC study meeting paper OQE85-79, The institute of Electronics Information and Communication Engineers". The quantum well semiconductor laser comprises a first cladding layer, a multi-quantum well active layer, a second cladding layer, a SiO.sub.2 film and a cap layer successively provided on a substrate. The quantum well semiconductor laser further comprises an n-electrode provided on the SiO.sub.2 film to contact with the cap layer, and a p-electrode provided on the back surface of the substrate. In this quantum well semiconductor laser, Zn preferential diffusion regions are provided on both sides of a stripe excitation region to disorder the multi-quantum well active layer, so that the structure converted to a uniform composition in which a refractive index and a bandgap energy of the multi-quantum well active layer are low and wide, respectively, is obtained. As a result, a high light output power is obtained in the quantum well semiconductor laser, because the absorption coefficient for a laser light in regions of the active layer in the vicinities of resonator facets is decreased due to the disorder.
However, the large optical cavity structure has a disadvantage of increase in threshold current and operation current. This results in the occurrence of the saturation of a light output power due to Joule heating, at lower output power than that limited by the catastrophic optical damage.
Further, the non-absorbing mirror laser and the window structure laser, in which the absorption coefficient in the active layer is decreased in the vicinities of the resonator facets, have a disadvantage of large variety in laser performance, because it is difficult to control the quality of the interface between the active layer and the buried layer and an impurity concentration of the excitation region and the regions in the vicinities of the resonator facets.
Still further, the laser device including the multi-quantum well active layer which is disordered in the regions of the resonator facets has a disadvantage of limited effect of light output power improvement, because of relatively light absorption coefficient ranging from 225 to 450 cm.sup.-1 in the vicinities of the resonator facets due to high carrier concentration of over 10.sup.19 cm.sup.-3 in the disordered region.